The present invention relates to an image display device preferably used for a view finder of a video camera or a head mounted type display or the like, and more particularly to a glasses type virtual image display device having a see-through function.
This application of the invention claims a priority based on Japanese Patent Application No. 2002-124824 filed in Apr. 25, 2002 in Japan. The earlier application is applied to this application by referring thereto.
An image display device used as a view finder of a video camera or a head mounted type display or the like has been hitherto proposed. As such an image display device, a virtual image display device formed by using a reflection type spatial light modulator is proposed.
As one example of this kind of image display device, an image display device is disclosed in the description of U.S. Pat. No. 5,596,451. The image display device described in this description has a polarized beam splitter cube 125 of a cubic form as shown in FIG. 1. The polarized beam splitter cube 125 has a polarized beam splitter surface 125E on a diagonal surface.
This image display device has an illuminating light source device 121 and a polarizer 123 opposed to a first surface 125A of the polarized beam splitter cube 125 which is disposed at an angle of 45° relative to the polarized beam splitter surface 125E. A reflection type spatial light modulator 122 is opposed to a second surface 125B of the polarized beam splitter cube 125 which is disposed at an angle of 45° relative to the polarized beam splitter surface 125E and at an angle of 90° relative to the first surface 125A. A quarter-wave plate 126 and a reflection mirror 127 are opposed to a third surface 125C of the polarized beam splitter cube 125 which is parallel to the second surface 125B.
In the image display device, beams of light emitted from the illuminating light source device 121 penetrate the polarizer 123, so that the beams become linearly polarized beams as S polarized beams relative to the polarized beam splitter surface 125E. The polarized beams are reflected on the polarized beam splitter surface 125E and polarized by 90° and the polarized beams reach the reflection type spatial light modulator 122. The reflected beams in which the polarized states are modulated in accordance with a display image are emitted from the reflection type spatial light modulator 122.
P polarized components of the reflected beams relative to the polarized beam splitter surface 125E penetrate the polarized beam splitter surface 125E, pass through the quarter-wave plate 126 and are reflected on the concave reflection surface of the reflection mirror 127. The reflected beams in the reflection mirror 127 pass through the quarter-wave plate 126 again, so that the reflected beams become S polarized beam relative to the polarized beam splitter surface 125E. The reflected beams 128A that reach the polarized beam splitter surface 125E are reflected by the polarized beam splitter 125E and polarized by 90°, reach the pupil 131 of a human being and are observed in an observation area 130.
As another example of the image display device, an image display device is disclosed in the description of U.S. Pat. No. 5,886,822. The image display device described in this description has, as shown in FIG. 2, a polarized beam splitter cube 301 having a cubic form like the above-described image display device. The polarized beam splitter cube 301 has a polarized beam splitter surface 324 on a diagonal surface.
In the image display device, an optical wave guide 300 made of an optical medium is provided on a first surface of the polarized beam splitter cube 301 disposed at an angle of 45° relative to the polarized beam splitter surface 324 to optically come into tight contact with the polarized beam splitter cube 301. At the terminal end part of the optical wave guide 300, a first lens 360 is disposed to optically come into tight contact with the optical wave guide 300. Further, an image display element 320 is opposed to the first lens 360.
In this image display device, image display beams 308 outgoing from the image display element 320 are incident on the optical wave guide 300 through the first lens 360, reflected on the polarized beam splitter surface 324, and then, emitted through a second lens 370 and reach the pupil 500 of an observer. In this image display device, a virtual image is formed by the first and second lenses 360 and 370.
In this image display device, since a large physical distance can be provided between the image display element and the virtual image forming lens, the image display element does not need to be provided just in front of the eyes of an observer. The degree of freedom in design is advantageously large.
As a still another example of the image display device, an image display device is disclosed in Japanese Patent Application Laid-Open No. 2001-264682. In the image display device described in the publication, as shown in FIG. 3, display lights L emitted from an image display element 201 are allowed to be incident on a prism 202. The display lights are allowed to be reflected a plurality of times between two reflecting surfaces 202a and 202b opposed to each other in this prism 202 and to be guided to an enlarging lens. The image display element 201 in the image display device serves to modulate intensity.
As the enlarging lens, a reflection type hologram lens 203 is employed. The reflection type hologram lens 203 forms a virtual image. That is, in the image display device, the display lights L emitted from the image display element 201 are incident on the prism 202, and then, internally reflected a plurality of times between the two opposed reflecting surfaces 202a and 202b. Then, the display lights L to which the reflection type hologram lens 203 gives a power for forming the virtual image are emitted from the prism 202 and reach the pupil 204 of an observer.
In this image display device, while the display lights repeat an internal reflection in the prism, the display lights are transmitted to the enlarging lens. Therefore, the image display device can conveniently decrease the thickness of an optical system more than the image display device shown in FIG. 2.
In the above-described image display devices, the image display device shown in FIG. 1 has such problems as described below.
Initially, the beams from the illuminating light source device 121 partly pass through a fourth surface 125D of the polarized beam splitter cube 125 as stray beam 128B as shown by broken lines in FIG. 1 and reach the pupil 131. The stray beams 128B constitute noise for image data displayed by the reflection type spatial light modulator 122 and deteriorate the contrast of the displayed image.
In the optical system of the image display device shown in FIG. 1, the maximum value of an exit pupil diameter or a display angle of view of the optical system is restricted depending on the size of the polarized beam splitter cube 125 for polarizing the reflected beams 128A for displaying an image toward the pupil 131 side. In the image display device, in order to increase a value such as the maximum value of the exit pupil diameter or the display angle of view while a constant eye relief is maintained, the polarized beam splitter cube 125 needs to be enlarged. When the polarized beam splitter cube 125 is enlarged, the thickness of all the optical system is inconveniently increased and the weight is also increased.
The polarized beam splitter cube 125 forming the image display device shown in FIG. 1 is hardly produced and a production cost thereof is high. Accordingly, the manufacture of the entire image display device is difficult and the manufacture cost thereof is undesirably increased.
The image display device shown in FIG. 2 has such problems as described below.
Initially, the image display device shown in FIG. 2 has a structure that the maximum value of an exit pupil diameter or a display angle of view of an optical system is restricted depending on the size of the polarized beam splitter cube 301 for reflecting the image display beams 308. In this image display device, in order to increase a value such as the maximum value of the exit pupil diameter or the display angle of view while a constant eye relief is maintained, the polarized beam splitter cube 301 and the optical wave guide 300 need to be enlarged. When the polarized beam splitter cube 301 and the optical wave guide 300 are enlarged, the thickness of all the optical system is inconveniently increased and the weight is also increased.
The polarized beam splitter cube forming the image display device is hardly produced and a production cost thereof is high. Accordingly, the manufacture of the entire image display device is difficult and the manufacture cost thereof is undesirably increased.
The image display device shown in FIG. 3 has such problems as described below.
The image display device shown in FIG. 3 uses a decentered optical system which is more suitable for a thin structure than the image display devices using coaxial optical systems shown in FIGS. 1 and 2. However, the reflection type hologram lens 203 can not be arranged so as to be parallel to the pupil 204 not to increase an aberration, that is, cannot be arranged so as to be perpendicular to the optical axis of the pupil 204. Accordingly, in the image display device, in order to increase the exit pupil diameter or the display angle of view of the optical system, the thickness of the prism 202 is increased and the weight is also increased.
In the optical system forming the image display device shown in FIG. 3, the reflection type hologram lens 203 on which the image display lights are incident and that is inclined with respect to the optical axes of the image display lights has the power for forming the virtual image. That is, this optical system is the decentered optical system.
A quantity of eccentricity in this optical system, that is, the angle of incidence or the angle of emergence of the image display lights to the reflection type hologram lens 203 is an angle exceeding 10° in a medium forming the prism 202. In the optical system having such a quantity of eccentricity, an enormous quantity of eccentric aberration is generated. It is difficult only for the reflection hologram lens 203 to correct the eccentric aberration.
In this image display device, a high resolving power, for instance, an MTF (Modulation Transfer Function) not lower than 20% cannot be ensured for a spatial frequency of 50 lines/mm.